Hi María,
sounds like this could be done via a restart simulation. Run your initial simulation without the pile (i.e. the pile material group has the same material properties as the sourrounding material) and use the final result as the input mesh for the second simulation where you can specify different material properties for the pile.
There is a benchmark about restart simulations, although it is probably not that helpful link
The corresponding prj files are here.
Best regards,
Florian